Sequence of one α- and two β-tubulin genes of Tetrahymena pyriformis: Structural and functional relationships with other eukaryotic tubulin genes

Abstract

Macronuclear DNA of the ciliate Tetrahymena pyriformis contains only one size class of fragments coding for α-tubulin, αTT. We have isolated αTT from a partial plasmid library, using Chlamydomonas reinhardtii α-tubulin gene as a probe. This gene as well as the two β-tubulin genes, βTT1 and βTT2, have been sequenced. None of these genes contains introns and all use TGA as the stop codon. In the coding region of the two β-tubulin genes, there are several TAA and TAG stop codons that probably code for glutamine. The codon usage is very biased. Regions flanking the tubulin coding sequences are A + T-rich (75%) and quite different among themselves. In these regions there are several putative transcription-regulatory sequences. Nuclear transcripts begin and terminate at multiple sites. The β-tubulin proteins differ only in two amino acid residues. Primary structure of Tetrahymena tubulins as well as their hydropathy indexes show a high degree of homology with tubulins from other organisms. Two-dimensional electrophoretic analysis of the ciliary tubulins shows the presence of eight α-tubulins and four β-tubulins. The α-tubulins migrate faster than the β-tubulins, in contrast with what happens with brain tubulins. We suggest that there are several α- and β-tubulin isoforms and the migratory inversion observed may be due to post-translational modifications.